This invention relates generally to gate valves and more particularly to an improved valve seat structure for expanding gate valves. Heretofore, problems have been experienced with expanding gate valve structures in maintaining the upstream and downstream seat elements in parallel so that the expanding gate assembly can expand and create an effective seal with both of these seat elements. In the higher pressures (greater than 5,000 psi), it becomes extremely difficult to achieve an upstream seal due to the large amount of operating torque required to expand the gate assembly into the upstream seat element. Earlier valve seat structures utilize a plastic insert extending from the front face of the seat element a specified distance so that the plastic insert conforms to the flatness and parallelism of the gate assembly prior to its expansion into metal-to-metal contact against the seat element. However, these plastic inserts have insufficient strength to adequately operate at pressures greater than 5,000 psi. Other earlier structures utilize floating seat elements with front and rear seals which create a pressure energized upstream seal. However, these structures cause the gate assembly to back wedge. In other words, once the gate assembly has collapsed and the gate is in motion, the valve seat structure continues to exert a load or pressure on the segment. When the load overcomes the frictional resistance associated with moving the segment relative to the gate, the segment will wedge up the gate causing the gate assembly to expand prematurely. This is, of course, very undesirable.
In light of these problems, it would be highly desirable to provide an improved valve seat structure that compensates for out-of-parallelism and misalignment between the gate assembly and the upstream valve seat when the gate assembly is in the expanded condition. The valve seat structure of this invention performs this function without tending to cause back wedging of the gate assembly.